Predicting Rogue Waves

Predicting Rogue Waves

Tracking monster waves: Scientists from the German Space Agency use satellite data indicating average wave heights to calculate where rogue waves could occur.

Scientists from the German Space Agency say they have mapped incidents of extremely large waves, known as rogue waves, using synthetic aperture radar (SAR) satellite data, and will soon publish a massive wave atlas for the first time. Such waves can mysteriously surge 100 feet (or about the height of a 12-story building) and sink massive cargo ships in their wake.

The scientific community has been slow to validate the existence of rogue waves, which are loosely defined as having an individual crest height that is more than twice the average height of surrounding waves. Besides sailors’ eyewitness accounts and discernible ship damage, evidence of abnormally large waves, which are extremely rare, has been limited to only a few rare photos, ocean oil-platform readings, and very occasional buoy data readings.

Difficulties in detecting the phenomenon are posed by the limited areas that traditional wave-height-measuring systems cover. There are a relatively small number of buoys or oil platforms collecting such data, and they are rarely deployed in remote oceans and seas where rogue waves are thought to be more likely to appear. During the very rare circumstances when high waves do surge against buoys or oil platforms, wave-height sensors are often damaged. Buoys that can withstand the impact without being destroyed are often incapable of measuring freak waves that are twice the average crest heights. Measurements of rogue waves, instead, are often indicated as mistaken readings.

Now, with two years of data from European Space Agency (ESA) satellites in tow, German Space Agency scientists say they are able to offer a rogue-wave map by taking advantage of the satellites’ global coverage. The researchers used data from two ESA satellites that orbited the earth 12 times a day and took SAR images every 200 kilometers for two years. SAR is a remote sensing radar system with which images are created by tracking how emitted radio waves bounce off the earth’s surface.

Using data from more than one million images, the German Space Agency scientists then calculated ocean surface heights with equations and models they created. The researchers pinpointed rogue waves up to 30 meters tall in the North Atlantic Ocean near Rockall (an island off the southwest coast of Greenland), in the North Pacific, in the Pacific Ocean southwest of Australia, and near the Cape Horn.

Unlike spectral data emitted by satellites, which enable only average wave sizes to be determined, the German Space Agency research group’s SAR data calculations are more precise, the researchers say. They have been able to determine individual wave heights around the world for the first time, says Susanne Lehner, who was involved in the research.

“Spectral analyses only give an average over an area,” Lehner says. “We derived the surface areas, the wave heights from top to bottom, and crest heights of individual waves.”

By using the German Space Agency’s rogue-wave atlas pinpointing where the monster waves have appeared in the past, real-time weather forecasts could, in theory, help prevent many accidents and deaths on the open ocean by indicating when and where dangerously high rogue waves might occur. Indeed, ship sinkings and ensuing deaths caused by the phenomenon are probably more numerous than officially recorded, Lehner says, given the large number of vessels that simply disappear without a trace every year.